1. Field of the Invention
The present invention relates to a thermoplastic multilayer composite having improved barrier action against diffusion of chemicals, and improved adhesion between the layers of the composite.
2. Discussion of the Background
While polyamides and polyesters are individually useful for a variety of end products, they are unsuitable for a number of specific uses. For example, polyamides show poor resistance to weathering, since they age under exposure to light and absorb atmospheric moisture, leading to discoloration, deterioration in mechanical properties and warping phenomena.
Although polyamides have good mechanical properties, in particular a good toughness, they provide poor barrier properties. In particular polar substances migrate easily through polyamides. This is especially a problem in fuel lines which transport alcohol-containing fuel.
Polyesters, on the other hand, exhibit good resistance to weathering and excellent barrier properties against both polar and non-polar media. However, they generally show unacceptable impact properties such as notched impact strength. Thus, polyesters cannot be used in cases where combinations of properties such as excellent barrier action, high temperature resistance and good rigidity, are desired.
German Patent 38 27 092 describes a multilayer pipe which has, from the inside outward, layers of polyamide, polyvinyl alcohol, polyamide and polyester. The polyester is used only as a thin outer layer, in order to achieve a higher resistance to short-term heat deformation. However, it is well known in the field of polymer composites that the majority of polymers, such as polyamides and polyesters, are incompatible with one another. Thus, the typical result of most multilayer composites is that the laminate layers show no adhesion to one another. An intimate bond between the individual polymer layers is absolutely essential for industrial uses, in order to avoid delamination.
Multilayer composites of particular polyamide mixtures and various other thermoplastics, such as polyethylene terephthalate, are known from EP-A-287 839. Conventionally, in order to achieve necessary adhesion, an adhesion promoter is introduced between the laminate layers. Examples of such adhesion promoters include functionalized polyolefins, functionalized ethylene/vinyl acetate copolymers, ethylene/acrylate copolymers, ionomers, polyalkylene oxide/polyester block copolymers, derivatives of carboxymethyl cellulose and blends of these polymers with polyolefins.
It has now been found that these adhesion promoters do not give a long-term intimate bond, especially in a polyamide/polyester system. Because the adhesion promoters are not sufficiently resistant to thermal deformation and solvents, the adhesion which is achieved is usually lost under conditions of heating or under the action of solvents. Additionally, these composites fail when exposed to shear forces due to cold flow of the adhesion promoter.